Analog circuit design differs from digital circuit design. Digital circuits prefer operating with binary numbers, a logical one or logical zero, represented by a pair of voltage levels—a logical high voltage level and a logical low voltage level or a digital signal. That is, a digital integrated circuit operates with discrete (binary) signals. The voltage levels between the logical high voltage level and the logical low voltage level were typically considered to be noise and usually unwanted in digital circuits. A digital integrated circuit typically utilizes the capabilities of Boolean logic gates to perform functions. Thus, the performance of a digital integrated circuit is less sensitive to placement, orientation, and the physical structure of transistor switches.
In contrast, analog circuitry operates using an analog signal over a range of voltages of an analog signal that can be between a maximum level and a minimum level. That is, an analog integrated circuit is an IC that operates with inter-module communication signals that are continuous rather than discrete. An analog integrated circuit has analog circuitry that exploits and utilizes the full spectrum of capabilities exhibited by individual low-level components, such as transistors, diodes, resistors, capacitors and inductors. The performance of an analog integrated circuit is very sensitive to the layout (placement, orientation, and physical pattern) of its low-level components. For example, noise immunity of an analog circuit and isolation of digital circuits from analog circuits can be important in the layout of analog circuitry.
Traditionally, analog circuitry has been manually laid out into its semiconductor mask layers for semiconductor manufacturing. This is because a human layout designer typically was experienced in analog circuit layout with prior knowledge, experience, and skill as to how the devices in an analog circuit were laid out so as to provide better performance and/or better noise immunity. The human layout designer could make choices up front prior to laying out the analog circuitry.
However, the number of choices that can be made up-front prior to layout are limited. Moreover, a human layout designer requires considerable time to layout an entire analog circuit chip or an entire mixed signal chip with both analog and digital circuitry. It is desirable to speed up the process of laying out analog circuitry, reduce the costs of designing analog circuits, and provide the capability of having additional choices in the criteria for an analog circuit layout.